1. Field of the Invention
The present invention relates to a sheet stacking device, an image forming system, and a sheet stacking method, and more particularly concerns a sheet stacking device that aligns and stacks sheets of a recording medium such as paper, recording paper, transfer paper, or transparency (hereinafter simply referred to as “sheets”) delivered into the sheet stacking device when the sheets are discharged, an image forming system including the sheet stacking device and an image forming apparatus such as a copier, a printer, a facsimile, or a digital multifunction peripheral, and a sheet stacking method performed by the sheet stacking device.
2. Description of the Related Art
Conventionally, sheet processing apparatuses that perform various processing, e.g., postprocessing such as alignment, stapling, folding, and bookbinding, on sheets discharged from an image forming apparatus are widely known and used. Hereinafter, such a sheet processing apparatus that performs post processing is referred to as a sheet post processing apparatus. In recent years, variety of sheets desired to be processed by this type of sheet post processing apparatus has become noticeably wide. In particular, it has become more common to perform printing using a color image forming apparatus on a sheet of coated paper (hereinafter, “coated paper”) that produces a visually-superior image for a brochure, a leaflet, or the like. Coated paper generally has the following properties:
1) high surface smoothness;
2) high inter-sheet clinging force; and
3) low stiffness (Clark method).
These properties can make coated paper less favorable in terms of sheet stackability.
A technique utilizing a return roller for preventing such unfavorable stacking and stacking discharged sheets at a normal position is already known.
Meanwhile, an apparatus that utilizes a pressing member for preventing sheets from being misaligned on a sheet discharge tray is also already known. As an example of an apparatus that uses such a pressing member, a technique disclosed in Japanese Laid-open Patent Application No. 2004-284786 is known.
According to this technique, a sheet stacking device including an output tray, onto which image-formed sheets are to be discharged and stacked, and a pressing member that presses a trailing end portion in a direction, in which the sheets are discharged onto the output tray, is configured as follows. The output tray is movable a preset amount in a direction perpendicular to the sheet discharging direction to perform offset output. The pressing member is moved in synchronization with an offset operation of the output tray.
The conventionally-employed return roller can move back a sheet to a normal position as described above; however, the return roller cannot prevent a phenomenon that a subsequent sheet electrostatically clings to a preceding sheet and pushes out the preceding sheet. Furthermore, when a sheet discharge tray is shifted for offset sorting, an aligned state of the sheets in the sheet discharge tray cannot be maintained with the return roller.
To maintain the aligned state of the sheets, a combination of the pressing member disclosed in Japanese Laid-open Patent Application No. 2004-284786 and the return roller can be employed. The combination of the return roller and the pressing member allows, even when the sheet discharge tray is shifted, holding sheets at the normal position by moving back the sheets to the normal position with the return roller and causing the pressing member to hold the sheets at the position.
However, arranging the return roller and the pressing member in the sheet discharging unit requires space for accommodating them and space for mounting drive mechanisms that drive the return roller and the pressing member, respectively. When the sheet discharging unit is required to have such space, it naturally follows that the sheet discharging unit is increased in size.
Under the circumstances, there is a need for a technique for preventing a sheet from being pushed out by another sheet or misaligned due to electrostatic inter-sheet clinging using a compact device.